The invention is concerned with a method and an apparatus for the post-treatment of exhaust gas, particularly for lean-burn engines in motor vehicles, e.g., direct-injection diesel engines and gasoline engines, and with ensuring the full functioning of NOx storage catalytic converters in gasoline and diesel engines and particle filters in diesel engines.
With the oxidation catalytic converter located in the exhaust pipe of a modern gasoline or diesel engine using direct injection, SOx deposits in the oxidation catalytic converter impair the desired NO2 formation even to the point of destroying the effectiveness of the catalytic converter system. With NOx storage catalytic converters, NO2 is required for the accumulation process. With particle filters that operate using the CRT (continuously regeneration trap) method, NO2 is required for the continuously-occurring oxidation regeneration process of the soot particles. When sulphur contaminates the NOx storage catalytic converter, the desired NO2 accumulation is reduced by SOx deposits in the NOx adsorber resulting from the sulphur in the fuel until the effectiveness of the system is destroyed. This sulphur compound can be broken down by regenerating the storage catalytic converter by briefly applying elevated exhaust-gas temperatures (a temperature above 650xc2x0 C. is used in gasoline direct-injection engines). The realization of such exhaust-gas temperatures in diesel engines is not considered promising according to the related art. Particle filters that function according to the CRT method mentioned hereinabove require exhaust-gas temperatures that exceed 230xc2x0 C. for the continuously-occurring regeneration process. These conditions cannot always be met with direct-injection diesel engines. Consequently, the filter can become severely overloaded, which can destroy the particle filter.
In the process of regenerating NOx storage catalytic converters, CO resulting from the hydrocarbon in the fuel must be added, and, at the same time, a rich composition of exhaust gas (xcex less than 1) must be produced. With diesel engines, however, it is atypical for the hydrocarbons (HC) required for regeneration to be provided by means of internal processes, due to the principles involved; it is also extremely crucial and associated with considerable reductions in fuel economy. Process-gas flow rates are a great deal higher with the diesel engine than with the gasoline engine. As a result, the temperatures required for regeneration cannot be reached across the entire operating range.
Likewise, providing a xe2x80x9crichxe2x80x9d composition of exhaust gas post-combustion is also a problem with diesel engines, because an oxidation catalytic converter is required to form CO, an exhaust-gas temperature profile is not entirely sufficient, and cycles with rich exhaust gas can only be achieved using a by-pass system.
The object of the invention is to prevent the hereinabove-mentioned difficulties associated with the post-treatment of exhaust gas in modern lean-burn engines, particularly gasoline and diesel engines with direct injection in motor vehicles, and to provide a method and an apparatus for the post-treatment of exhaust gas produced by an internal combustion engine in such a fashion that the exhaust-gas temperature is raised as necessary, and the exhaust-gas quality is improved overallxe2x80x94especially under certain operating conditions of the internal combustion enginexe2x80x94while not making the engine acoustics worse, and while making regeneration of a storage catalytic converter and/or a particle filter possible at regular intervals and/or after sulphur poisoning at the oxidation stages of an NOx storage catalytic converter and particle filter.
According to an essential aspect, with the method according to the invention for the post-treatment of exhaust gas, a hydrolysis unit for obtaining hydrogen is provided that is connected to a water tank as well as a metering device that is designed to meter the hydrogen delivered to the untreated exhaust gas and/or to the exhaust gas treated by means of an oxidation catalytic converter as a function of a demand for hydrogen occurring at certain operating states and/or catalytic converter functions.
In an exemplary embodiment of the method, the quantity of hydrogen required in each case can be produced on demand, i.e., discontinuously, in the hydrolysis unit, and can then be made available directly for metering.
In an alternative exemplary embodiment of the method, a hydrogen tank can be provided that serves to provide intermediate storage for a certain quantity of the hydrogen produced by the hydrolysis unit.
The size of the hydrogen tank and, therefore, the quantity of the hydrogen stored for the interim, can thereby be designed so that it suffices to heat and regenerate an NOx storage catalytic converter.
When the connecting pipes between the metering device and the hydrolysis unit are designed accordingly, the tank can be represented by the inner lumen of the pipeline.
Preferably, the temperature of the untreated exhaust gas, the xcex value and, in addition, certain operating states of the catalytic converter system, are registered to meter the hydrogen to be delivered.
In the case of a diesel engine, especially with direct injection, the addition of hydrogen to the exhaust gas is activated when hydrocarbon cannot be produced using internal processes.
In the case of a gasoline engine, especially one with direct injection, the addition of hydrogen to the exhaust gas is activated when the engine operating point at the moment does not allow hydrocarbon to be provided using internal processes at a sufficient temperature.
The method according to the invention can be used to restore a sufficient conversion rate after sulphur poisoning at the oxidation stages of an NOx storage catalytic converter or a particle filter by regenerating the oxidation stages of the storage catalytic converter or the particle filter by means of hydrogen reduction. Regeneration by means of adding hydrogen can always be activated when a decrease in the conversion rate of the NOx storage catalytic converter or the particle filter is registered.
When an internal combustion engine operates under low-load conditions and temperature is therefore a crucial factor, adding hydrogen in accordance with the invention can raise the exhaust gas temperature in order to guarantee that the regeneration conditions are met during low-load operation of the engine when a particle filter is employed.
In an apparatus for the post-treatment of exhaust gas produced by an internal combustion engine, especially in a motor vehicle, that attains the object described hereinabove, the following are provided: a hydrolysis unit and a metering device connected to it via a hydrogen line for the metered addition of hydrogen to the untreated exhaust gas and/or to the exhaust gas treated by means of the oxidation catalytic converter, and a control and regulating unit that are functionally connected to the hydrolysis unit and the metering device, in order to control or regulate the production of hydrogen in the hydrolysis unit and the metering device as a function of certain operating states of the internal combustion engine and registered parameters of the exhaust-gas system.
The metering device is preferably a metering and shutoff valve.
The control/regulating unit preferably comprises a catalytic converter monitoring function that is functionally connected to an exhaust-gas sensor system.
The above-mentioned and further advantageous features of the method according to the invention and the apparatus according to the invention are explained in the subsequent description of preferred exemplary embodiments of the method according to the invention and the apparatus according to the invention, with reference to the drawings.